This invention relates to insecticide microcapsules and, more particularly, to the microencapsulation of methomyl, 5-methyl-N-[(methylcarbamoyl)oxythioacetamide] to form a controlled release microcapsule.
Although methomyl is an important broadspectrum insecticide for use on many vegetables, field crops, fruit crops and ornamentals, it can decompose in as few as three days after application, depending on the field conditions, and can be phytotoxic to some plants, e.g., cotton, under certain conditions. Thus, efforts have been directed to developing techniques to improve the field performance of methomyl. Among general methods of microencapsulation most closely related to the process of the present invention are the following:
British Pat. No. 1,371,179, discloses a process for encapsulating water-immiscible material within a polymer shell which involves dispersing a water-immiscible phase that contains at least one organic polyisocyanate in an aqueous phase at a temperature of 20.degree. to 90.degree. C. to achieve hydrolysis-polymerization-encapsulation.
British Pat. No. 1,513,225 teaches that the polymer capsules of British Pat. No. 1,371,179 can be treated with ammonia or amines to remove unreacted isocyanate.
U.S. Pat. No. 3,959,464, alternatively, teaches the preparation of microcapsules of methyl or ethyl parathion contained within a wall of cross-linked polyamide-polyurea prepared in aqueous media.
None of these processes are practical for methomyl because methomyl exhibits poor solubility in the monomers used and a high solubility in water, especially at elevated temperatures e.g. as employed in British Pat. No. 1,371,179. In order to obtain high concentrations of methomyl in microcapsules, it is necessary that the material be retained in the organic dispersed phase until the polymer skin is formed.